1. Field of the Invention
The present invention is directed to a radio-frequency antenna for a magnetic resonance system having a number of antenna rods and two end rings of the type wherein the antenna rods are regularly spaced around an antenna axis and are connected at their respective rod ends to the end rings.
2. Description of the Prior Art
In an antenna of the above type, the antenna rods can be arranged in different ways. Each antenna rod can proceed essentially parallel to the antenna axis and have a rod spacing in a middle region of the antenna axis that is smaller than a end ring spacing that at least one of the end rings exhibits from the antenna axis in the region of this antenna rod. Alternatively, each antenna rod can form an angle of inclination with the antenna axis and have a rod spacing from the antenna axis at its end lying closer to the antenna axis that is smaller than the end ring spacing exhibited by the end ring that is connected to that rod end, in this region of this antenna rod.
Such radio-frequency antennas are generally known as birdcage resonators, particularly when the antenna rods are parallel to the antenna axis. Capacitors are arranged in the end rings and/or in the antenna rods in these. The radio-frequency antenna is tuned such that it forms a resonant circuit at a predefined or predefinable operating frequency of the radio-frequency antenna. German PS 197 32 783 is an example of such an arrangement. A similar disclosure can be derived from JP-A2000 166 895.
Nuclear spins of an examination subject (often a human being) are excited into resonance due to the magnetic fields emitted by such a radio-frequency antenna. The resonant signals are then acquired and interpreted. The acquisition of the resonance signals can potentially ensue with the same antenna.
At least one whole-body antenna is present in a magnetic resonance system. The rod spacing therein usually lies between 25 and 35 cm. Other components are usually present. These particularly include local coils. Local coils are employed in order to achieve a noticeably improved signal-to-noise ratio in the imaging than with a global or whole-body antenna. Local coils for the head or extremity examination of a human are often constructed according to the same principle as whole-body antennas. A relatively strong coupling of the antennas, and thus a deterioration of the signal-to-noise ratio, occurs due to this fact, namely the geometrically similar structure.
In order to keep the coupling of the antennas within limits, the whole-body antenna is detuned in the case of reception by the local coil or coils. To this end, detuning circuits that deactivate the whole-body antenna as warranted are installed in the whole-body antenna. For example, the deactivation can be achieved with diode switches that short the capacitors and thus shift the resonant frequency of the radio-frequency antenna. At least eight detuning switches in the antenna are required for this purpose in the case of a conventional radio-frequency antenna having, for example, sixteen antenna rods.
Further, the patient is irradiated with a high-frequency magnetic field during the examination. The patient thus absorbs radio-frequency power. Although the power absorbed by the patient is monitored, only the globally absorbed power averaged over the weight of the patient can be identified. Significantly, higher electrical and magnetic fields locally, particularly in regions wherein the patient extends very close to the radio-frequency antenna in the transmission case, so that higher power absorption can occur locally.
An object of the present invention is to provide a radio-frequency antenna of the type suitable for magnetic resonance systems with which interactions of the radio-frequency antenna with other antennas are reduced and unacceptable heating of the patient is avoided.
In a whole-body antenna (i.e. an antenna wherein the rod spacing lies between 25 and 35 cm), this object is achieved in an antenna of the initially-described type wherein the difference between the end ring spacing and the rod spacing lies between 5 and 15 mm.
The inductance of the appertaining end ring becomes lower by increasing the end ring spacing or spacings. The detuning elements therefore function better since their impedance is transformed to a lower-impedance at the influencing location. The appertaining end ring is also at a geometrically greater distance from the structures of the local coils.
By avoiding any increased end ring spacings in the local coil, the coupling due to geometrically similar structures is reduced. The signal-to-noise ratio of the local coils thus is reduced to only a very slight degree.
Moreover, the appertaining end ring is then at a greater distance from the patient. The magnetic fields that are generated by currents in the end ring are thus lower. A localized absorption of radio-frequency power in the patient is reduced.
Due to the lower inductance of the end ring larger capacitances are required in order to set the same resonant frequency. The electrical fields that the capacitors exhibit thus also are reduced. A reduction of the local absorption of the radio-frequency power in the patient also occurs as a result.
The above measures become more pronounced the greater the end ring spacing is compared to the rod spacing.
Preferably, the antenna rods and the end rings are radially surrounded by a radio-frequency shield at the exterior.
As a rule, the radio-frequency shield has a shield spacing from the antenna axis in the region of the antenna rod under consideration. An especially good effect due to the increase in the end ring spacing is achieved when the difference between the end ring spacing and the rod spacing amounts to at least 15%, preferably 20 through 40%, of the difference between the shield spacing and the rod spacing.
The radio-frequency shield can be symmetrically or asymmetrically arranged relative to the antenna axis.
The outlay for the detuning of the radio-frequency antenna also can be reduced due to the lowering of the appertaining end ring. In particular, it suffices when the radio-frequency antenna still has only (exactly) two detuning circuits with which the radio-frequency antenna can be detuned.
Conventionally, the detuning circuits are installed into the radio-frequency antenna itself. In particular, they are arranged in the connecting regions of the end rings to the antennas rods. In the inventive radio-frequency antenna, in contrast, it is possible for the detuning circuits to be connected to the feed cable for the radio-frequency antenna.
When the radio-frequency antenna is mounted on the exterior of a carrying tube, then the structural arrangement of the individual components of the radio-frequency antenna can be realized in an especially simple way.
In an embodiment wherein the antenna rods form at least two sub-structures that are rotated relative to one another, each of the sub-structures is regularly arranged around the antenna axis and the sub-structures comprise rod spacings that differ from one another. The radio-frequency antenna that is optimized even further.
As a rule, the antenna rods or the sub-structures are circularly arranged around the antenna axis. Alternatively, they can be arranged elliptically, for example, around the antenna axis.
The end rings are usually symmetrically arranged relative to the antenna axis. Alternatively, they can be asymmetrically arranged relative to the antenna axis.
The connection of the antenna rods to the one end ring or to the end rings can be effected by the antenna rods proceeding radially outwardly toward the appertaining rod end given an essentially parallel course of the antenna rods relative to the antenna axis, the antenna rods can be alternatively bent radially outwardly in the region of their rod ends or can proceed gradually radially outwardly from the middle region toward the rod ends.
Alternatively, the connection of the antenna rods to the one end ring or to the end rings is effected by the appertaining end ring being conducted radially inward in its connecting region to the antenna rods.